1. Field of the Invention
The present invention is related to a piezoelectric actuator having an electromechanical converting element which moves a moving member by utilizing inertia force from an inertia member. More particularly, the present invention is related to the control of a drive signal for driving the electromechanical converting element of the piezoelectric actuator.
2. Description of the Related Art
FIG. 1 (prior art) is a diagram illustrating a conventional piezoelectric actuator. Such a convention piezoelectric actuator is disclosed, for example, in Japanese Laid-Open Patent Publication Number Sho 63-299785. Referring now to FIG. 1, a piezoelectric actuator 10 includes a moving member 11 and an inertia member 13, with a piezoelectric member 12 therebetween. Piezoelectric member 12 has one end mounted to moving member 11 and another end mounted to inertia member 13. Moving member 11 is in contact with a fixed member 14 through a friction surface 11a. Fixed member 14 does not make contact with piezoelectric member 12 or inertia member 13.
FIGS. 2(A), 2(B), 2(C), 2(D) and 2(E) are diagrams illustrating the operation of the conventional piezoelectric actuator illustrated in FIG. 1. Referring now to FIGS. 2(A), 2(B), 2(C), 2(D) and 2(E), one asymmetric waveform cycle of a drive signal 50 is applied to piezoelectric member 12.
In FIG. 2(A), various forces affect moving member 11, wherein these forces are related to the weight, M, of moving member 11, the weight, m, of inertia member 13 and the dynamic friction coefficient, .mu., between moving member 11 and fixed member 14. FIG. 2(A) also illustrates the displacement, x, of piezoelectric member 12, and the displacement, y, of moving member 11. At the point of time "ta" in FIG. 2(A), the pulse of drive signal 50 has not yet been applied to piezoelectric member 12, and a drive force has not been produced. At the point of time "tb" in FIG. 2(B), a portion of the pulse of drive signal 50 with a large voltage increase rate is applied to piezoelectric member 12. This large voltage increase rate causes piezoelectric member 12 to expand in correspondence with the applied voltage. At this time, the expansion rate of piezoelectric member 12 is large and steep. Therefore, a reaction force of the shock inertia 52 generated from inertia member 13 is added to moving member 11, and that reaction force overcomes the static friction force of friction surface 11a. As a result, moving member 11 moves in the direction of arrow 54.
At the point of time "tc" in FIG. 2(C), piezoelectric member 12 stops expanding and, because displacement is not produced, moving member 11 stops moving.
At the point of time "td" in FIG. 2(D), a portion of the pulse of drive signal 50 with a small voltage reduction rate is applied to piezoelectric member 12. Piezoelectric member 12 contracts in correspondence with the applied voltage. At this time, the contraction rate of piezoelectric member 12 is small and gentle due to the small voltage reduction rate. Therefore, the reaction force 56 of inertia member 13 becomes smaller than the static friction force of friction surface 11a. As a result, moving member 11 does not move. For this situation to occur, the voltage reduction rate of drive signal 50, as illustrated in FIG. 2(D), must be sufficiently small as compared to the voltage increase rate of drive signal 50, as illustrated in FIG. 2(B).
At the point of time "te" in FIG. 2(E), a pulse of drive signal 50 is not being applied to piezoelectric member 12, and piezoelectric actuator 10 enters the same initial state as in FIG. 2(A).
In this manner and as illustrated by FIGS. 2(A), 2(B), 2(C), 2(D) and 2(E), moving member 11 moves in accordance with one asymmetric waveform cycle of drive signal 50 applied thereto. Specifically, when an asymmetric drive signal is used to drive piezoelectric member 12, the first half of a pulse of the drive signal has a large voltage increase rate so that the reaction force of the shock inertia generated from inertia member 13 is added to moving member 11. This reaction force overcomes the static friction force of friction surface 11a. As a result, moving member 11 moves.
The latter half of the pulse of the drive signal has a small voltage reduction rate. Then, the reaction force of inertia generated from inertia member 13 is added to moving member 11. However, the reaction force is smaller than the static friction force of friction surface 11a. As a result, moving member 11 does not move. In this manner, the movement of moving member 11 is controlled by one asymmetric waveform cycle of a drive signal.
If the weight of inertia member 13 is smaller than the weight of moving member 11, the amount of drive caused by one asymmetric waveform can be extremely minute. This is an important, beneficial aspect of piezoelectric actuator 10 since it is desirable to cause movable body 11 to move by very small amounts. However, the amount of movement of moving member 11 can undesirably change if the friction coefficient of friction surface 11a changes between moving member 11 and fixed member 14. Such change in the friction coefficient can occur from changes in temperature, humidity or other such factors. As a result, a predetermined or set amount of movement of moving member 11 per cycle of the drive signal may undesirably change. Therefore, the amount of movement of moving member 11 for a specific number of cycles of the drive signal may change, and the time required for moving member 11 to move a predetermined distance may change.
Moreover, the surface roughness of friction surface 11a can significantly change during movement, thereby undesirably changing the amount of movement of moving member 11 per cycle of the drive signal.
FIG. 3 illustrates a piezoelectric actuator as disclosed in Japanese Laid-open Patent Publication Number Hei-4-207982. The piezoelectric actuator illustrated in FIG. 3 attempts to solve the above-described problems by intentionally changing the friction coefficient of the friction surface. Referring now to FIG. 3, the piezoelectric actuator has a moving member 206 with a first inertia member 208 and a second inertia member 210 connected thereto. First inertia member 208 is connected to moving member 206 via a first piezoelectric member 207. Second inertia member 210 is connected to moving member 206 via a second piezoelectric member 209. A respective sinusoidal wave (symmetric waveform) is applied to first piezoelectric member 207 and a different, respective sinusoidal wave is applied to second piezoelectric member 209. First piezoelectric member 207 expands and contracts, in accordance with the sinusoidal wave applied to first piezoelectric member 207, to change the friction coefficient of a drive surface 205. In this manner, moving member 206 is driven when the friction coefficient becomes low. The amount of drive of moving member 206 is controlled by changing the phase difference of the two sinusoidal waves applied, respectively, to first piezoelectric member 207 and second piezoelectric member 209.
However, it is difficult and complicated to control the phase of the sinusoidal waves applied to first piezoelectric member 207 and second piezoelectric member 209. In addition, as described above, the piezoelectric actuator illustrated in FIG. 3 moves moving member 206 by changing the friction coefficient of the friction surface. However, it is difficult to determine the appropriate friction coefficient and to properly change the friction coefficient. Moreover, a piezoelectric actuator as illustrated in FIG. 3 requires two piezoelectric bodies (as opposed to only one piezoelectric member) and two inertia bodies (as opposed to only one inertia member). Therefore, a piezoelectric actuator as illustrated in FIG. 3 is relatively expensive and difficult to manufacture.